Remove Adapter autoloads in favor of require

Adapters must be eager loaded to ensure they are defined
before they are used as namespacing.

cf6a074a1c (diff-41f2b3509d33e1c65bb70ee0ec7a2eea)

lib/active_model/serializer/adapter/null.rb

@@ -1,5 +1,11 @@
-class ActiveModel::Serializer::Adapter::Null < ActiveModel::Serializer::Adapter
+module ActiveModel
+  class Serializer
+    class Adapter
+      class Null < Adapter
         def serializable_hash(options = nil)
           {}
         end
+      end
+    end
+  end
 end